For we maintain that a man is justified by faith apart from works of the law.
28. We then conclude, etc. He now draws the main proposition, as one that is incontrovertible, and adds an explanation. Justification by faith is indeed made very clear, while works are expressly excluded. Hence, in nothing do our adversaries labor more in the present day than in attempts to blend faith with the merits of works.
From all this Paul infers that it is in vain to look for justification by the works of the law, and that it is to be had only by faith, which is the point he has been all along proving, from Rom 1:17, and which he lays down (Rom 3:28) as the summary of his discourse, with a quod erat demonstrandum - which was to be demonstrated.
Commenting on Romans 3:19-31
Verse 28. Therefore. As the result of the previous train of argument. That a man. That all who are justified; that is, that there is no other way. Is justified by faith. Is regarded and treated as righteous, by believing in the Lord Jesus Christ. Without the deeds of the law. Without works as a meritorious ground of justification.